Polymerization of styrene and other styrenic monomers may take place by a variety of techniques such as bulk or solvent polymerization, emulsion polymerization or suspension polymerization. In each of these techniques, polymerization results from a free radical induced, chain-propagation mechanism. Stated in simpler terms, styrenic monomers form polymers when contacted with free radicals. Accordingly, in order to cause styrenic monomers to polymerize, it is necessary to combine these monomers with free radicals.
There are two known techniques for providing free radicals for the polymerization of styrenic monomers. One such technique is to simply heat the styrenic monomers to a temperature, e.g., above 120.degree. C., which causes the generation of free radicals by thermally induced interactions among these styrenic monomers. This technique is often referred to as thermal initiation. Another technique is distinguished from thermal initiation in that it involves combining catalytic quantities of free radical initiators with the styrenic monomers. These free radical initiators generate free radicals by thermally induced decomposition. Accordingly, both the "thermally induced" technique and the initiator induced technique are thermal in the sense that they both involve heating of the polymerization system. However, the initiator induced technique generally takes place at a lower temperature than the thermally initiated technique, the temperature of the initiator induced technique being such that free radicals are generated substantially from initiator decomposition as opposed to the interaction of styrenic monomers, particularly in the initial stages of the polymerization.
Free radicals not only initiate polymerization, the rate at which polymerization proceeds increases as the concentration of free radicals in the styrenic monomers increases. Also, the concentration of free radicals generated by a given quantity of styrenic monomers or free radical initiators increases as temperature increases. Accordingly, increasing temperature generally tends to increase rates of reaction at least immediately after the temperature is increased. However, if the temperature is held constant the polymerization rate will tend to decrease over time, because as the polymerization proceeds less and less styrenic monomer and/or free radical initiator are left to generate more free radicals. Accordingly, in order to drive the polymerization to completion, it is known to increase the temperature as the reaction proceeds. It is also known to use a plurality of free radical initiators which decompose at different temperatures. When such a plurality of free radical initiators is used, the temperature may be maintained at one temperature until one free radical initiator is substantially decomposed, and then the temperature may be increased to decompose the remaining initiators.
Free radicals could also be generated by incrementally incorporating free radical initiators with the styrenic monomers as the polymerization progresses. Such an incremental incorporation is known in the case of bulk or emulsion polymerizations. However, in the case of suspension polymerizations, special problems are presented with incremental additions of free radical initiators, due to the manner in which styrenic monomers are polymerized in such suspension polymerizations.
In suspension polymerizations, styrenic monomers are present in a suspension of droplets dispersed throughout a continuous water medium. Although the free radical initiators used in such suspension polymerizations are soluble in styrenic monomers, these initiators are substantially insoluble in water. Accordingly, it is most customary to incorporate all of the initiator or initiators with the styrenic monomers before the suspension polymerization is started.
The Doak U.S. Pat. No. 2,907,756, the entire disclosure of which is incorporated herein by reference and relied upon, suggests that an incremental addition of initiators may be made in a suspension polymerization of styrene but only during the initial stages of the polymerization. More particularly, it is stated that it is necessary to add all of the initiator before polymerization has progressed to such an extent that intimate mixing of catalyst and styrene monomer is precluded. Specifically, addition of initiator is terminated when the suspension reaches a temperature of about 115.degree. C. (See Column 6, lines 8-14 of the Doak U.S. Pat. No. 2,907,756). Referring to Table II of the Doak U.S. Pat. No. 2,907,756 (See Column 7, lines 7-22), the percent conversion of the polymerization reaches about 49% at the time the suspension reaches a temperature of about 115.degree. C.
It is understandable why the Doak U.S. Pat. No. 2,907,756 would seem to discourage one from adding initiator to the suspension during the later stages of polymerization. At these stages, two barriers to prevent mixing of initiators and monomers are present, namely, (1) the continuous water phase, in which the initiator is insoluble, and (2) the substantial amount of polymer which has formed. However, in spite of these barriers it has unexpectedly been found that initiator can indeed be effectually added to the later stages of a suspension polymerization, when such addition is made in accordance with the present invention.